This invention relates to a charge transfer device constituting an image pickup apparatus, and more particularly to a charge transfer device provided with means for controlling the photoelectric conversion characteristics so as to obtain an output made suitable for the image pickup apparatus in accordance with the intensity of light from a foreground subject.
The charge transfer device has already been proposed as a "Charge Coupled Device" (which is hereinafter abbreviated to "CCD") or "Bucket Brigade Device" (which is hereinafter abbreviated to "BBD"). The "CCD" is proposed by W. S. Boyle et al in the treatises "Charge Coupled Semiconductor Device" and "Experimental Verification of the Charge Coupled Device Concept" appearing at pages 587 to 600 of "The Bell System Technical Journal" issued April 19, 1970.
For convenience of explanation of this invention, the fundamental construction and the operation principle of a prior art "CCD" will hereinafter be described by reference to FIGS. 1 and 2. FIG. 1-A shows the construction of the "CCD" of an N channel type-3 phase driving system. As shown in FIG. 1-A, an insulating film 2 is provided on a P type semiconductor substrate 1. On the insulating film 2 are arranged a plurality of electrodes 3a, 4a, 5a, 3b, 4b, 5b, ... 3n, 4n and 5n in the order mentioned. Electrode conductors .phi.1, .phi.2 and .phi.3 are connected to the electrodes (3a, 3b, ... 3n), (4a, 4b, ... 4n) and (5a, 5b, ... 5n), respectively. Each electrode conductor is supplied with a voltage having such a waveform as is illustrated in FIG. 2 from a driving voltage source 6. The maximum level V.sub.2 and the minimum level V.sub.0 of each voltage waveform are both positive and bear the relationship of V.sub.0 &lt; V.sub. 2. T1, T2 and T3 represent one frame period, integration period and transfer and readout period, respectively. In the integration period T.sub.2, when a voltage V.sub.2 is applied to the electrode conductor .phi.1 and a voltage V.sub.0 to the remaining electrode conductors .phi.2 and .phi.3, potential wells 7a, 7b, ... 7n are produced in the semiconductor substrate below the integration electrodes 3a, 3b, ... 3n (FIG. 1-A). When, under this condition, the optical image of a foreground subject (not shown) is focussed on the semiconductor substrate 1 via a lens not shown, electron-hole pairs 8a, 8b, ... 8n corresponding to the brightness of the optical image are produced in the substrate 1. The minority carriers of the electron-hole pairs, namely, electrons 9a, 9b, ... 9n are shifted by diffusion to the potential wells in the proximity of the electron-hole pairs and are integrated therein.
When, after completion of said integration, the voltages V.sub.1, V.sub.2 and V.sub.0 (V.sub.0 &lt; V.sub.1 &lt; V.sub.2) are applied to the electrode conductors .phi.1, .phi.2 and .phi.3, respectively, such a potential distribution as is indicated by a numeral 10 of FIG. 1-B is obtained with the result that the integrated electrons 9a, 9b, ... 9n start to be shifted to the potential wells produced below the electrodes 4a, 4b, ... 4n as shown by arrows. When the voltage V.sub.0 is applied to the electrode conductors .phi.1 and .phi.3 and the voltage V.sub.2 to the electrode conductor .phi.2 (FIG. 1-C) after the substantial completion of electron shifting, transference of the electrons 9a, 9b, ... 9n to the potential wells 11a, 11b, ... 11n is completed. Through applying a voltage having such a waveform as is shown in, for example, FIG. 2 to the electrode conductors .phi.1, .phi.2 and .phi. 3 during the transfer and readout period T3 the integrated electrons 9a, 9b, ... 9n are sequentially drawn out as video signals from the output terminal 16 by readout means including, for example, a diode 13 consisting of an N.sup.+ region and an electrode 12, DC source 14 and output resistor 15. A reference numeral 18 denotes an amplifier. The video signals from the output terminal 16 are transmitted to the image display means of the image pickup apparatus via amplifier means thereof, and the displayed image is monitored there.
In some cases, the ratio of the brightness of the lightest portion of a foreground subject to be displayed as an image of a television camera to the brightness of the darkest portion of the foreground subject, that is, the contrast ratio (or contrast range) indicates more than 10.sup.3 : 1. However, the contrast ratio of a reproduced television image is normally limited to 20 : 1 to 30 : 1. In other words, in the television, the reproduced image bearing part of the contrast ratio of a foreground subject can only be obtained.
However, where it is desired to make a reproduced image easy to see, the image reproduction has to be so performed as to cause the foreground subject to fall for the most part within said reproducible contrast ratio. To this end, for example, adjustment of the iris diaphragm of an image pickup lens is generally carried out. That portion of the foreground subject which has a brightness exceeding the upper limit of said reproducible contrast ratio causes production of an image signal output having a large amplitude, so that excess modulation is caused in the transmitter, or the video amplifier is saturated to cause the amplifying function thereof to be temporarily stopped. Consequently, contrast depression is required in effecting the image pickup.
As the methods for effecting the contrast depression there are known (1) the method of performing the image pickup through an optical element whose light transmissivity is varied in accordance with the intensity of light incident into the camera from a foreground subject, (2) the method of effecting the contrast depression within an image signal processing circuit, and (3) the method of effecting the contrast depression by an image pickup element itself. As the last-mentioned method (3) there are known the method of utilizing the knee pattern of the photoelectric conversion characteristic of an image orthicon and the method of utilizing the Anti-Comet Tail Operation of the "Plumbicon."
When the contrast depression is carried out by the image pickup element, it is desirable that the intensity of an incident light corresponding to the knee point of the photoelectric conversion characteristic of said element is controllable and that the inclination of the photoelectric conversion characteristic portion beyond said knee point of the photoelectric conversion characteristic (the contrast compression ratio varies with said inclination) is controllable.
Where image pickup is performed by three pickup tubes of R.G.B. (Red; Green; Blue), the respective operating points (the respective levels of the knee points) of the three pickup tubes do not coincide with each other. Accordingly, the respective levels of the output signals of the three pickup tubes are not equalized. Under these circumstances, the level correction is usually performed by controlling the gain of the video amplifier. When said knee point levels and inclinations of said three pickup tubes are different, an undesirable coloring takes place in the reproduced image owing to the contrast depression. In other words, a good color reproducibility is not achieved. The foregoing description was made by taking the image pickup tube as an example. But a charge transfer device having satisfactory means for satisfactorily controlling the photoelectric conversion characteristic has not yet been proposed.
Accordingly, the object of the invention is to provide a charge transfer device having new satisfactory means for controlling the photoelectric conversion characteristic as desired.